1. Field of the Invention
The present invention relates to an inverter for supplying a fluorescent lamp and the like with drive voltage and it particularly relates to a drive system including the inverter.
2. Description of the Related Art
In recent years, flat-screen LCDs growing in size and flatter in thickness are in wide use replacing the CRT displays. In the LCD, a plurality of cold cathode fluorescent lamps (hereinafter referred to as CCFL) or external electrode fluorescent lamps (hereinafter referred to as EEFL) are placed on the back surface of LCD panels on which images are displayed, and they emit light as backlight.
An inverter (DC/AC converter), which boosts DC voltage of, for example, about 12 V and outputs it as AC voltage, is used to drive CCFL or EEFL. The inverter converts the current flowing to CCFL, to a voltage and then feeds it back to a control circuit. Based on this fed-back voltage, the inverter controls the on and off of switching elements. For example, Patent Document 1 discloses a technology for driving fluorescent lamps by such an inverter.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-323994.
To generate a boosted AC voltage by the inverter, energy needs to be stored by supplying intermittently the switching voltage to a primary coil of a transformer. To supply the switching voltage to the primary coil of a transformer, a method can be employed wherein four switching transistors are configured by a so-called H-bridge circuit or full-bridge circuit so that switching voltage is supplied across the primary coil.
1. In a case when the switching voltage is generated by using such an H-bridge circuit, the timing at which each switching transistor turns on and off has a significant effect on the efficiency of an inverter. Also, when a pair of transistors, connected in series between an input voltage and ground among the switching transistors that configure the H-bridge circuit, turn on simultaneously, a through current flows. Hence, a switching control needs to be performed by providing dead time.
2. Also, in order to switch the on and off of the transistors, gate potential of MOSFETs or base current of bipolar transistors needs to be varied. In the case of MOSFETs, a gate capacitance must be charged or discharged if the gate potential is to be changed. Accordingly, the transistor size becomes larger and therefore the time required for the gate potential to transit becomes longer. As a result, there are cases where a certain delay time occurs from when OFF is specified to a transistor until when the gate potential has transited and turns off. As delay is caused in ON-OFF timing, a pair of transistors, connected in series among the switching transistors constituting the H-bridge circuit, may turn on simultaneously and thereby a through current may flow.